1. Field of the Invention.
The present invention relates to the transmission of electromagnetic energy. In particular it relates to antennas designed for flush mounted operation.
2. Description of the Prior Art.
The limitations of thin aperture antennas have been theoretically defined by D. Rhodes in the IEEE Transactions on Antennas and Propagation, May, 1972, pages 318-325. Significant results of his derivations show that the depth of a cavity does not influence gain or bandwidth. However, the achievement of a cavity antenna of minimal depth, a very desirable design objective, is complicated by the generation of a large amount of capacitance. Such capacitance interferes with the transfer of electromagnetic energy into free space.
Previous design art for removing the capacitance associated with shallow cavities has consisted of parasitic elements (stubs, tuning screws) in the E plane of the cavity. Often, in these cavities, no space exists for such parasitic elements. An alternate design approach has been to modify the E plane by narrowing some portion of the antenna aperture. This method tends to narrow the antenna frequency bandwidth and squint the radiation pattern for a circular polarized source. Another design method eliminates capacitance by not extending the cavity beyond the probe (IEEE Transactions on Antennas and Propagation, January, 1975, pages 1-7). This method restricts window design for flush mounted applications.
The present invention achieves a very thin cavity antenna the transfer of energy from which overcomes the capacitance introduced. A chamber placed at the top of the cavity having a metallic probe located therein serves to minimize the effect of capacitance due to the minimal cavity depth.